An Overview of Optic Fiber
The earliest attempts to communicate via light undoubtedly go back
thousands of years. Early long distance communication techniques, such as
“smoke signals”, developed by native North Americans and the Chinese
were, in fact, optical communication links. A larger scale version of this
optical communication technique was the “optical telegraph” deployed
in France and elsewhere in the late 18th century. The “optical
telegraph” was a series of tall towers that passed along messages at a
rate of a few words per minute by means of large semaphore flags that could be
manipulated to spell out words. However, the development of fiber optic
communication awaited the discovery of TIR (Total Internal Reflection) and a
host of additional electronic and optical innovations.
1.1 Historical Development of Fiber Optic
Jean-Daniel Colladon, a 38-year-old Swiss professor at University of
Geneva, demonstrated light guiding or TIR for the first time in 1841. He wanted
to show the fluid flow through various holes of a tank and the breaking up of
water jets. However, in the lecture hall the audience could not see the flowing
water. He solved the problem by collecting and piping sunlight through a tube
to the lecture table. The light was focused through the water tank and was made
to incident on the edge of the jet at a glancing angle. TIR trapped the light
in the liquid forcing it to follow the curved path until the water jet broke
up. Instead of traveling in a straight line, the light followed the curvature
of the water flow. Colladon later on wrote, “I managed to illuminate the
interior of a stream in a dark space. We have discovered that this strange
arrangement offers in results one of the most beautiful, and most curious
experiments that one can perform in a course on Optics.” (15,Oct 1842).
Colladon demonstrated light guiding in water jets through a number of public
performances to the urban intelligentsia of Paris. Auguste de la Rive, another
Geneva Physicist, duplicated Colladon’s experiment using electric arc light.
Colladon designed a spectacular device using arc light for Conservatory of Arts
and Science of Paris in 1841, October.
Colladon sent a paper to his friend Francois Arago who headed the
French Academy of Sciences and edited its journal Comptes Renedes. Arago
recalled that Jacques Babinet, a French specialist in Optics had made similar
demonstrations in Paris. He focused candle light on to the bottom of a glass
bottle as he poured a thin stream of water from the top. TIR guided the light
along the jet. Arago asked Babinet to write down his work, but Babinet did not
think that the work is very important. Yet he made a comment that “the
idea also works very well with a glass shaft curved in whatever manner and I
had indicated that (it could be used) to illuminate the inside of the mouth
(15, Oct. 1842). After sending his letter to Arago, Babinet never returned to
guiding of light before he died in 1872. The earliest attempts to communicate
via light undoubtedly go back thousands of years. Early long distance
communication techniques, such as “smoke signals”, developed by
native North Americans and the Chinese were, in fact, optical communication
links. A larger scale version of this optical communication technique was the
“optical telegraph” deployed in France and elsewhere in the late 18th
century. The “optical telegraph” was a series of tall towers that
passed along messages at a rate of a few words per minute by means of large
semaphore flags that could be manipulated to spell out words. However, the
development of fiber optic communication awaited the discovery of TIR (Total
Internal Reflection) and a host of additional electronic and
optical innovations.
In 1870, John Tyndall, using a jet of water that flowed from one container
to another and a beam of light, demonstrated that light used internal
reflection to follow a specific path. As water poured out through the spout of
the first container, Tyndall directed a beam of sunlight at the path of the
water. The light, as seen by the audience, followed a zigzag path inside the
curved path of the water. This simple experiment, illustrated in Figure 1.1,
marked the first research into the guided transmission of light.
Figure
1.1. Typical Early TIR (Total Internal Reflection) Demonstration.
William
Wheeling, in 1880, patented a method of light transfer called “piping light.”
Wheeling believed that by using mirrored pipes branching off from a single
source of illumination, i.e. a bright electric arc, he could send the light to
many different rooms in the same way that water, through plumbing, is carried
throughout buildings today. Due to the ineffectiveness of Wheeling’s idea and
to the concurrent introduction of Edison’s highly successful incandescent light
bulb, the concept of piping light never took off.
Fiber optic technology experienced a phenomenal rate of progress in the
second half of the twentieth century. Early success came during the 1950’s with
the development of the fiberscope. This image-transmitting device, which used
the first practical all-glass fiber, was concurrently devised by Brian O’Brien
at the American Optical Company and Narinder Kapany (who first coined the term
“fiber optics” in 1956) and colleagues at the Imperial College of Science and
Technology in London. Early all-glass fibers experienced excessive optical
loss, the loss of the light signal as it traveled through the fiber, limiting
transmission distances. This motivated scientists to develop glass fibers that
included a separate glass coating. The innermost region of the fiber, or core, was used to transmit
the light, while the glass coating, or cladding, prevented the
light from leaking out of the core by reflecting the light within the boundaries
of the core. This concept is explained by Snell’s Law which states that the
angle at which light is reflected is dependent on the refractive indices of the
two materials – in this case, the core and the cladding. The lower refractive
index of the cladding (with respect to the core) causes the light to
be angled back into the core as illustrated in Figure 1.2 [2].The fiberscope
quickly found application inspecting welds inside reactor vessels and
combustion chambers of jet aircraft engines as well as in the medical field.
Fiberscope technology has evolved over the years to make laparoscopic surgery
one of the great medical advances of the twentieth century.
Figure
1.2. Optical Fiber with Cladding.
1.2 Application of Optic Fiber in the Real World
The U.S. military
moved quickly to use fiber optics for improved communications and tactical
systems. In the early 1970’s, the U.S. Navy installed a fiber optic telephone
link aboard the U.S.S. Little Rock. The Air Force followed suit by developing
its Airborne Light Optical Fiber Technology (ALOFT) program in 1976. Encouraged
by the success of these applications, military R&D programs were funded to
develop stronger fibers, tactical cables, ruggedized, high-performance
components, and numerous demonstration systems ranging from aircraft to
undersea applications.
Commercial
applications followed soon after. In 1977, both AT&T and GTE installed
fiber optic telephone systems in Chicago and Boston respectively. These
successful applications led to the increase of fiber optic telephone networks.
By the early 1980’s, single-mode fiber operating in the 1310 nm and later the
1550 nm wavelength windows became the standard fiber installed for these
networks. Initially, computers, information networks, and data communications
were slower to embrace fiber, but today they too find use for a transmission
system that has lighter weight cable, resists lightning strikes, and carries more
information faster and over longer distances.
Today, DWDM
technology continues to develop. As the demand for data bandwidth increases,
driven by the phenomenal growth of the Internet, the move to optical networking
is the focus of new technologies. At this writing, over 800 million people have
Internet access and use it regularly. That’s over 12% of the entire world’s
population of 6.4 billion people. The world wide web already hosts over 350
million domain names, 8 billion web pages (that’s only the visible, indexed,
Internet, the invisible Internet is estimated to be up to 100 times larger),
and according to estimates people upload more than 3.5 million new web pages
everyday. The Internet dominates traditional voice communication as shown in Figure
1.3.
The
important factor in these developments is the increase in fiber transmission
capacity, which has grown by a factor of 200 in the last decade. Figure 1.4 illustrates
this trend.
Broadband
service available to a mass market opens up a wide variety of interactive
communications for both consumers and businesses, bringing to reality
interactive video networks, interactive banking and shopping from the home, and
interactive distance learning. The “last mile” for optical fiber goes from the
curb to the television set top, known as fiber-to-the-home
(FTTH) and fiber-to-the-curb
(FTTC), allowing video on
demand to become a reality.
 |
Figure
1.3. Projected Internet Traffic Increases.
 |
Figure 1.4. The Growth of Optical Fiber Transmission
Capacity.
1.3 Optical Fibers as a
Communication Channel
The role of a communication
channel is to transport the optical signal from transmitter to receiver without
distorting it. Most light wave systems use optical fibers as the communication
channel because silica fibers can transmit light with losses as small as 0.2
dB/km. Even then, optical power reduces to only 1% after 100 km. For this
reason, fiber losses remain an important design issue and determine the
repeater or amplifier spacing of a long-haul light wave system. Another
important design issue is fiber dispersion,
which leads to broadening of individual optical pulses with propagation.
If optical pulses spread
significantly outside their allocated bit slot, the transmitted signal is
severely degraded. Eventually, it becomes impossible to recover the original signal
with high accuracy. The problem is most severe in the case of multimode fibers,
since pulses spread rapidly (typically at a rate of ~10 ns/km) because of different speeds
associated with different fiber modes. It is for this reason that most optical communication
systems use single-mode fibers. Material dispersion (related to the frequency dependence
of the refractive index) still leads to pulse broadening (typically <0.1
ns/km), but it is small enough to be acceptable for most applications and can
be reduced further by controlling the spectral width of the optical source. Material
dispersion sets the ultimate limit on the bit rate and the transmission distance
of fiber-optic communication systems.
1.3.1 Optical
Transmitters
The role of an optical transmitter is to convert the
electrical signal into optical form and to launch the resulting optical signal
into the optical fiber. Figure 1.5 shows the block diagram of an optical
transmitter. It consists of an optical source, a modulator, and a channel
coupler. Semiconductor lasers or light-emitting diodes are used as optical sources
because of their compatibility with the optical-fiber communication channel.
The optical signal is generated by modulating the optical carrier wave.
Although an external modulator is sometimes used, it can be dispensed with in
some cases, since the output of a semiconductor optical source can be modulated
directly by varying the injection current. Such a scheme simplifies the
transmitter design and is generally cost-effective. The coupler is typically a
mi crolens that focuses the optical signal onto the entrance plane of an
optical fiber with the maximum possible efficiency.
Figure 1.5. .
The launched power is an important design parameter. One can
increase the amplifier (or repeater) spacing by increasing it, but the onset of
various nonlinear effects limits
how much the input power can be increased. The launched power is often
expressed in “dBm” units with 1 mWas the reference level [4]. The general
definition is:
Where,
dBm is decibel meter, 1 mW is 0 dBm, 1 µW corresponds to ?30 dBm. The launched power is rather
low (<?10dBm) for
light-emitting diodes but semiconductor lasers can launch powers ~10 dBm. As light-emitting diodes are
also limited in their modulation capabilities, most lightwave systems use
semiconductor lasers as optical sources. The bit rate of optical transmitters
is often limited by electronics rather than by the semiconductor laser itself.
With proper design, optical transmitters can be made to operate at a bit rate
of up to 40 Gb/s. Chapter 3 is devoted to a complete description of optical
transmitters [4].
1.3.2 Optical Receivers
An optical receiver converts the optical signal received at the
output end of the optical fiber back into the original electrical signal.
Figure 1.6 [4] shows the block diagram of an optical receiver. It consists of a
coupler, a photo-detector, and a demodulator. The coupler focuses the received
optical signal onto the photo-detector. Semiconductor photodiodes are used as photo-detectors
because of their compatibility with the whole system. The design of the
demodulator depends on the modulation format used by the lightwave system. The
use of FSK and PSK formats generally requires heterodyne or homodyne
demodulation techniques discussed in Chapter 10. Most lightwave systems employ
a scheme referred to as “intensity modulation with direct detection” (IM/DD).
Demodulation in this case is done by a decision circuit that identifies bits as
1 or 0, depending on the amplitude of the electric signal. The accuracy of the
decision circuit depends on the SNR of the electrical signal generated at the photo-detector
[4].
Figure 1.6. Components of an
Optical Receiver.
1.4 Wavelength Division
Multiplexing
Main
article wavelength division multiplexing
(WDM) is the practice of multiplying the available capacity of an optical fiber
by adding new channels, each channel on a new wavelength of light. This
requires a wavelength division multiplexer in the transmitting equipment and a
de-multiplexer (essentially a spectrometer)
in the receiving equipment. Arrayed waveguide gratings are commonly
used for multiplexing and de-multiplexing in WDM. Using WDM technology now
commercially available, the bandwidth of a fiber can be divided into as many as
160 channels to support a combined bit rate into the range of terabits
per second.
The
effect of dispersion increases with the length of the fiber, a fiber
transmission system is often characterized by its bandwidth-distance product, often expressed in units of MHz×km. This value is a
product of bandwidth and distance because there is a trade off between the
bandwidth of the signal and the distance it can be carried. For example, a
common multimode fiber with bandwidth-distance product of 500 MHz×km could
carry a 500 MHz signal for 1 km or a 1000 MHz signal for 0.5 km .Through a
combination of advances in dispersion management, wavelength-division multiplexing,
and optical amplifiers, modern-day optical fibers can carry information at
around 14 Terabits per second over 160 kilometers of fiber. Engineers are
always looking at current limitations in order to improve fiber-optic
communication.
The
per-channel light signals propagating in the fiber have been modulated at rates
as high as 111 gigabits per second by NTT, although 10 or
40 Gb/s is typical in deployed systems. Each fiber can carry many
independent channels, each using a different wavelength of light (wavelength-division multiplexing
(WDM)). The net data rate (data rate without overhead bytes) per fiber is the
per-channel data rate reduced by the FEC overhead, multiplied by the number of
channels (usually up to eighty in commercial dense WDM
systems as of 2008). The current laboratory fiber optic
data rate record, held by Bell Labs in Villarceaux, France, is multiplexing 155
channels, each carrying 100 Gb/s over a 7000 km fiber [4].
1.4.1 Total
Internal Reflection
When a ray of light passes from one transparent
medium to another, for example at the surface of a pool of water, it generally
bends at the boundary .This phenomenon is well known: a stick poked into water
appears to bend. The bending of the ray at the boundary is described by Snell’s
law, a simple relationship between the sine’s of the angles that the ray makes
on the two sides of the boundary. Mathematically, Snell’s law can be written
as:
n1sin?1=n2sin?2
Where, n1
is core refractive index and n2
is cladding refractive index, ?1
is incident angle and ?2
is refracted angle of the media on opposite sides of the boundary is shown in Figure
1.7.
Figure 1.7. Total Internal
Reflection.
The refractive
index of each medium is a number that characterizes the optical density
of the medium relative to a vacuum. It is a number that describes how much more
slowly light travels in the medium relative to its velocity in a vacuum. If n2 is less than
n1 this equation
restricts the angles at which a ray of light can get across the boundary. If
light is passing from medium 1 to medium 2 and the angle ?1 is greater than the critical angle, then the light can
not refract across the boundary (because ?1
can not be greater than 90o). The critical angle is the angle whose
sine is n2/n1.
When a ray of light strikes a boundary at an angle greater than the critical
angle it reflects, and does not cross. Although many devices that we use in
real life use this phenomenon, and although many natural optical phenomena
depend on its occurrence, we may not be aware of it.
Optical fibers use total internal reflection to keep
a light ray trapped within the denser glass of the center of a composite
cylindrical glass fiber, the core.
It is as if light rays are guided down the core of the fiber in a zigzag path
by a succession of total internal reflections at the boundary between the core
glass and the less dense glass surrounding it – the cladding, is shown in Figure 1.8.
Before discussing the advantages of optical
communication along such fibers, we must go back in time to discuss the
properties of glass, and how these properties had to be modified to make
optical fibers sufficiently transparent to make long distance light
transmission along them possible.
Figure
1.8. Internal Reflection Inside the Core.
Fiber Modes
The
concept of the mode is a general concept in optics occurring also, for example,
in the theory of lasers. An optical
mode refers to a specific solution of the wave equation that satisfies
the appropriate boundary conditions and has the property that its spatial
distribution does not change with propagation. The fiber modes can be
classified as guided modes, leaky modes, and radiation modes. As one might
expect, signal transmission in fiber-optic communication systems takes place
through the guided modes only. The following discussion focuses exclusively on
the guided modes of a step-index fiber. To take advantage of the cylindrical
symmetry, written in the cylindrical coordinates ?, ?, and z as
where,
for a step-index fiber of core radius a,
the refractive index n is of
the form
Single-Mode Fibers
Single-mode
fibers support only the HE11 mode, also known as the fundamental mode of the
fiber. The fiber is designed such that all higher-order modes are cut off at
the operating wavelength. The V parameter
determines the number of modes supported by a fiber. The cutoff condition of
various modes is also determined by V.
The fundamental mode has no cutoff and is always supported by a fiber. The single-mode condition is determined
by the value of V at which the
TE 01 and TM01 modes reach cutoff.
Dispersion in Single-Mode Fibers
Intermodal dispersion in multimode fibers leads to
considerable broadening of short optical pulses (~10 ns/km). In the geometrical-optics description, such broadening was
attributed to different paths followed by different rays. In the modal
description it is related to the different mode indices (or group velocities)
associated with different modes. The main advantage of single-mode fibers is that intermodal dispersion is absent simply because
the energy of the injected pulse is transported by a single mode. However,
pulse broadening does not disappear altogether.
The
group velocity associated with the fundamental mode is frequency dependent because
of chromatic dispersion. As a result, different spectral components of the
pulse travel at slightly different group velocities, a phenomenon referred to
as group-velocity dispersion (GVD),
intermodal dispersion, or
simply fiber dispersion. Intermodal
dispersion has two contributions, material dispersion and waveguide dispersion.
We consider both of them and discuss how GVD limits the performance of light wave
systems employing single-mode fibers.
Material Dispersion
Material
dispersion occurs because the refractive index of silica, the material used for
fiber fabrication, changes with the optical frequency ?. On a fundamental
level, the origin of material dispersion is related to the characteristic
resonance frequencies at which the material absorbs the electromagnetic
radiation. Far from the medium resonances, the refractive index n(?) is well approximated by the Sellmeier equation
where,
?i is the resonance frequency and Bj is the oscillator strength. Here n stands for n1
or n2, epending on whether the
dispersive properties of the core or the cladding are considered.
1.5
Fiber Bandwidth
The
concept of fiber bandwidth originates from the general theory of time-invariant
linear systems. If the optical fiber can be treated as a linear system, its input and output
powers should be related by a general relation
For
an impulse Pin(t) =? (t), where
? (t) is the delta function,
and Pout(t) = h(t). For this reason, h(t) is called the impulse
response of the linear system. Its Fourier transforms,
provides
the frequency response and is called the transfer
function. In general, |H(
f )| falls off with increasing f , indicating that the high-frequency components of the input
signal are attenuated by the fiber. In effect, the optical fiber acts as a band pass filter. The fiber bandwidth f3dB corresponds to
the frequency f = f3dB at which |H( f )| is reduced by a factor of 2 or by 3
dB:
That
f3dB is the optical bandwidth
of the fiber as the optical power drops by 3 dB at this frequency compared with
the zero-frequency response. In the field of electrical communications, the
bandwidth of a linear system is defined as the frequency at which electrical
power drops by 3 dB.
Optical
fibers cannot generally be treated as linear with respect to power [4].
However, this equation is approximately valid when the source spectral width is
much larger than the signal spectral width (V? >> 1).
In
that case, we can consider propagation of different spectral components
independently and add the power carried by them linearly to obtain the output
power. For a Gaussian spectrum, the transfer unction H( f ) is found
to be given by
where
the parameters f1 and f2 are given by
and
we used equation to introduce the dispersion parameters D and S.
For
lightwave systems operating far away from the zero-dispersion wavelength ( f1 << f2), the transfer
function is approximately Gaussian. By using equation with f << f2, the fiber bandwidth is
given by
If
we use ?D = |D|L?? from equation, we obtain the
relation f 3dB?D ? 0.188 between the fiber bandwidth and dispersion-induced pulse
broadening. We can also get a relation between the bandwidth and the bit rate B by using equation.
The
relation is B ? 1.33 f3dB and shows that the fiber bandwidth is an approximate
measure of the maximum possible bit rate of dispersion-limited lightwave
systems. Used to estimate f3dB
and its variation with the fiber length under different operating conditions.
For lightwave systems operating at the zero-dispersion wavelength, the transfer
function is obtained from equation by setting D = 0. The use of equation, then provides the following
expression for the fiber bandwidth.
The
limiting bit rate can be related to f3dB
by using equation and is given by B ? 0.574 f3dB. Again, the fiber bandwidth provides a measure of the dispersion
limited bit rate. As a numerical estimate, nsider a 1.55-?m lightwave system
employing dispersion-shifted fibers and multimode semiconductor lasers. By
using S = 0.05 ps/(km-nm2) and ?? = 1 nm as
typical values, f3dBL ? 32 THz-km. By contrast, the
bandwidth–distance product is reduced to 0.1 THz-km for standard fibers with D = 18 ps/(km-nm).
Fiber
Losses
Fiber
dispersion limits the performance of optical communication systems by
broadening optical pulses as they propagate inside the fiber. Fiber losses
represent another limiting factor because they reduce the signal power reaching
the receiver. As optical receivers need a certain minimum amount of power for
recovering the signal accurately, the transmission distance is inherently
limited by fiber losses. In fact, the use of silica fibers for optical
communications became practical only when losses were reduced to an acceptable
level during the 1970s. With the advent of optical amplifiers in the 1990s,
transmission distances can exceed several thousand kilometers by compensating
accumulated losses periodically. However, low-loss fibers are still required
since spacing among amplifiers is set by fiber losses. This section is devoted
to a discussion of various loss mechanisms in optical fibers.
Optical Fiber Communications System
The information-carrying capacity of an optical
fiber is far greater than it is for its competitors: wires, coaxial cables, and
microwave links. In addition, optical fibers are inexpensive to produce, do not
conduct electricity (which makes them immune to disturbance by lightning
storms, and other electromagnetic signals – except nuclear radiation), do not
corrode, and are of small size. The primary reason that optical fibers have
very much larger information-carrying capacity than other media, is that they
carry light: this might seem a trivially obvious observation but it has
fundamental significance [4]. The frequency of the light beams that
travel along optical fibers is in the vicinity of two hundred trillion cycles
per second (Hz). Compare this with the frequency of the latest generation of
personal communication service (PCS) cellular wireless systems – approximately
two billion cycles per second (2 GHz). Consider the frequencies that must be
transmitted for voice communications, which cover the range (bandwidth) from
about 50Hz to 20,000Hz (20kHz). Indeed, since there is very little need to
include the high frequencies for understandable voice communications, the
actual bandwidth needed is really only about 4 kHz. It is possible, in
principle, to carry about 50 billion voice conversations on a single laser beam
in an optical fiber. This capacity results from the very simple calculation:
2.1014Hz/4.103Hz=50.109.
The entire population of the Earth could be on the
phone on a single fiber at the same time. The corresponding capacity of a PCS
link is about 500,000 simultaneous voice channels. In practice, it has not
proven possible to achieve these maximum capacities, although in current links
the optical fiber wins by a huge margin. The domination of optical fiber as a
means for carrying information is apparent when we note that such fibers are
being manufactured worldwide at a rate of 2000 miles per hour! Whether an
optical or microwave link has the ability to use its full capacity depends on
the way in which the information is encoded, and how different messages are
mixed together (multiplexed)
without them all getting mixed up. To understand in more detail how this is
done, we must digress and discuss the nature of digital representations of
information and how this influences the way in which information, whether this
be voice, video, or computer data, is encoded and transmitted.
Fiber-optic communication is a method of transmitting information from
one place to another by sending pulses of light through an optical fiber.
The light forms an electromagnetic carrier wave
that is modulated
to carry information. First developed in the 1970s, fiber-optic communication systems have revolutionized
the telecommunications industry and have played a
major role in the advent of the Information
Age. Because of its advantages over electrical transmission,
optical fibers have largely replaced copper wire communications in core networks
in the developed world.
Everywhere on this planet hair-thin optical
fibers carry vast quantities of information from place to place. There are many
desirable properties of optical fibers for carrying this information. They have
enormous information-carrying capacity, are low cost, and possess immunity from
the many disturbances that can afflict electrical wires and wireless
communication links. The superiority of optical fibers for carrying information
from place to place is leading to their rapidly replacing older technologies.
Optical fibers have played a key role in making possible the extraordinary
growth in world-wide communications that has occurred in the last 25 years, and
are vital in enabling the proliferating use of the Internet.
Principal among these are the invention and development of the laser,
the growing appreciation that this might make optical communications
practically useful, the production of very pure glass, which was sufficiently
transparent that long distance transmission of light through glass fibers
became practical, and the digital revolution.
Optical communications systems have a long history. Ancient man
signaled with smoke and fire, often relaying messages from mountain top to
mountain top. However, this optical communication scheme had limited
transmission capacity. They could serve as a warning, as Queen Elizabeth the
First of England planned when she had a network of bonfires erected to be set
in the event of a seaborne invasion from Spain. The smoke signals transmitted
by Native Americans had the capacity to transmit various messages. Since the
end of the eighteenth century messages have been passed by semaphore – the use
of flags to indicate the transmission of one letter at a time. This form of
communication could transmit information at a rate of about one letter per
second over a direct line of sight, although messages could be relayed over
long distances. Such means of communication were not very secure: anyone in the
line of sight to the message sender could read the information (if he knew the
code). The message could also be intercepted and altered during the relay
process as the Count of Monte Cristo did to his advantage.
For optical communication to progress past
these early efforts, an information carrying channel had to be developed that
was reliable, inexpensive, and that could be used over long distances,
preferably at high rates of data transmission. The fundamental physical
phenomenon that makes this possible is called total internal reflection. This phenomenon causes light to reflect, rather than refract, when it attempts to cross
the boundary from one transparent optical medium to another of lower optical
density, at a sufficiently large angle. As early as 1854, in London, John
Tyndall demonstrated that light could be guided inside a transparent medium with
such a density discontinuity with its surroundings. He did this by showing
light being guided along a stream of water flowing from a container. His simple
demonstration proves that in the right circumstances light need not travel in straight lines.
1.7
Ring Topology of Optical Fiber Communication
In ring topology all the nodes are connected in a ring network. Each
node in the ring is feed from two sides and therefore, in the event of a single
break down of the link the traffic is not lost. A typical ring network is shown
in Figure 4.1.
1.7.1 Mesh Topology of Optical Fiber Communication
In Mesh topology each node in network is connected to every other node
in the network. So, each node is feed from several sides (n-1 connection, where
n is the number of nodes in the network). Hence, traffic will not be lost in
the event of multiple breaks of link also. In Mesh type network traffic is much
more protected, but this type of network is expensive because of the need to
construct multiple paths. Mesh network is shown in Figure 4.2 [24].
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1.7.2
Two-Fiber Unidirectional Path Protection Ring
Two-fiber
unidirectional Path Protection (PP) rings use the 1+1 protection mode. It is
composed of two optical fibers: one working fiber (S fiber) and one protection
fiber (P fiber). Two-fiber Unidirectional Path Protection Ring shown in Figure
4.3 [25].
Input
signal is simultaneously fed into both the working and protection fibers at the
Tributary Unit (TU). Receiving node selects the signal from either of the two
fibers, according to the quality of the signals.
Advantage
of the path protection rings:
·
Very short
switching time (Normally less than 15ms).
·
Service flow is
clear and simple.
·
Easy to operate,
administrate and maintain.
Disadvantages
of the path protection rings:
·
Time slots cannot
be reused.
·
Network capacity
is limited (STM-N).
 |
Figure
1.11. Two-fiber Unidirectional Path Protection Ring.
State Owners Optical Fiber Cable Network in Bangladesh
The first generation of optical communication system was deployed
during 1970’s at an operating wavelength of 850 nm due to availability of
semiconductor lasers at the same wavelength region and multimode fiber. With
progress in the development of lasers of longer wavelength and low loss single
mode fiber, the second generation of optical communication system started
during 1980s at about 1330 nm operating window of silica fiber and the third
generation of system operation started during 1990s with the deployment of
Erbium-doped fiber amplifiers (EDFAs) in the third operating window of 1550 nm
of silica fiber where the fiber loss is found to be minimum of 0.2 dB/km. Most
of the currently deployed fiber optic systems are based on the second and third
window of operating wavelength. The operating wavelengths of an optical fiber
link are determined by the attenuation and dispersion in fiber and availability
of optical communications devices and components. The low-loss transmission
windows centered about 1300nm and 1550nm. Each window covers a bandwidth of 10
to 15 THz. Each of the windows can be used for transmission of large number of
optical carriers operating at a bit rate of 10 Gb/s or more.
2.1 Optical Fiber in Bangladesh
Establishment of fiber optic links in Bangladesh began in 1986, along
with the installation of new digital switches. Starting with the optical fiber
link between Maghbazar and Gulshan telephone exchanges of Dhaka city, all
intra-city inter-exchange connections are now established through short
distance fiber optic links [5]. Deployment of optic fiber for long distance
transmission link was first started by Bangladesh Railway in mid 90 decade for
their internal communication system which was later rented to Grameenphone. Bangladesh
Railway deployed the optic fiber cable their railway tracks.
The first long distance inter-city optical communication link for
public telecommunication was deployed by BTTB (at present BTCL) in 2000-2001
connecting Dhaka and Chittagong cities with a STM-16 capacity optic fiber link
(“STM” is a standard of data transmission rate where STM-1 represents a bit
rate of 155 Mb/s, STM-4 of 622 Mb/s, STM-16 of 2.5 Gb/s and STM-64 of 10 Gb/s).
On the way from Dhaka to Chittagong, the link also connected Comilla and Feni
cities. Gazipur, B. Baria, Noakhali and Lakshmipur cities were also connected
to the optic fiber network at that time through some branch links. In the
year1999 BTTB took a very large project with the help of China to Digitalize
all the telephone exchanges in the district cities and at some important
upazila towns. Deployment of about 1000 km of optic fiber link was also there
in the scope of the project. Under the project Bogra to Panchagarh optic fiber
links were constructed in the year 2001-2002 which connected Rangpur, Sayedpur,
Dinajpur and Thakurgaon cities/towns on the way. Some branch links connected
Nilphamari, Lalmonirhat and Kurigram cities with the main optic fiber link
(Bogra to Panchagarh) of north Bengal. Several discrete links like Khulna to
Satkhira, Netrokona-Mymensingh-Shepur, Kushtia-Meherpur-Chuadanga,
Chittagong-Rangamati and Sirajganj-Pabna were also constructed under the
project in the year 2002.
The submarine optic fiber cable (SEA-ME-WE-4) landed at Cox’sbazar in Bangladesh
in the year 2006 and the optic fiber land cable link between Chittagong and
Cox’sbazar was also completed by BTTB in the same year. BTTB completed the work
of connecting Bogra city to Dhaka through optic fiber link in 2007 and on the
way Mymensingh, Tangail and Sirajganj cities were connected with the link.
Later in 2010 BTCL (former BTTB) connected Khulna and Sylhet cities through
optic fiber to Dhaka under a project aided by Korean government.
Private Telcos, specially the mobile phone operators started to deploy
their own optic fiber network from 2003. The biggest Mobile phone operated
leased the optic fiber network of Bangladesh Railway and also deployed new
optic fiber cable network in different part of the country. Other cell phone
operators such as Citycell, Robi and Banglalink also deployed huge optic fiber
cable network at different parts of the country. Bangladesh Telecom Regulatory
Commission (BTRC) issued NTTN (National Telecommunication Transmission Network)
license to an operator (Fiber@Home) in 2008. Fiber@Home started deploying inter
district optic fiber cable network and metropolitan optic cable network in some
large cities like Dhaka and Chittagong from 2009. Recently BTRC issued NTTN
license to another company (Summit group) and they have also started their
activity from this year. Among the private PSTN operators only Banglaphone have
some optic fiber cable network, mainly aerial type, at several parts of the
country.
Besides the optic links of Telecom operators stated above, there are
huge numbers of discrete optic fibers links in some big cities which are mainly
operated by the private Internet Service Providers (ISP) and Cable TV service
providers. Almost all of these links are aerial type and deployed in nonstandard
way using electricity supply line poles to hang the optic fiber cable. Recently
government has ordered all concern to remove all these optic fiber cables from
the road which are deployed in nonstandard way.
2.2 Bangladesh Railway Optical Fiber Networks
Bangladesh Railways (BR) owns the only fiber optics capacity in Bangladesh,
which was laid in the late eighties to establish a network that supported 1200
digital phones within the BR, Train Control System, Station to Station Phone
and Block Control System. The Fiber Optic Network, 1600 km long connecting
cities like Chittagong, Sylhet, Khulna, Rajshahi, Saidpur, Sirajganj, Mymensingh,
Comilla etc to Dhaka. The Fiber Optic Network was established with financial
assistance of NORAD. In 1997, in an act resonant with bravura market
prescience, Grameenphone (GP) leased the whole system for 20 years, by bidding
a price of $ 8 million. GP is a company limited by guarantee, in which Marubeni
(of Japan), Telenor (of Norway) and Grameenphone a company headquartered in the
US, have stakes. GP owns and operates Bangladesh’s first-ever Global Systems
Mobile (GSM) 900 cellular phone network. (Five other mobile phone companies
have been established in Bangladesh, namely, Citycell, Robi, Banglalink, Airtel
and State Owned Teletalk).
In anticipation of growing business and residential demand, the fiber
optics (FO) cable communication technology has been upgraded to Synchronous Digital
Hierarchy (SDH) equipment between Dhaka-Chittagong, Dhaka-Khulna, Dhaka-Bogra
and Dhaka-Sylhet. The capacity of the Fiber Optic Network where upgraded, is
now 7680 channels, but GP’s operating license with the government limits the
use of channels to 1920 channels. The speed of data transmission through
optical fiber network can be increased from 9.6 Kbps to 2 Mbps. Broadband
capacity is usually defined as equivalent to 2 Mbps. GP has applied for
permission to increase the present data capacity from 1920 channels to the
maximum of 7680 channels. The case for such an increase flows from the business
model of GP, which requires leveraging off the capacity to lease transmission
capacity to corporate clients and even government organizations, for example,
the Navy and Army. The full capacity between Bangladesh’s larger cities, if
brought into being, would also increase the number of mobile phones that can be
supported in cities other than Dhaka. The permission to expand the capacity was
requested by GP when the tender to upgrade was submitted, in 1998. GP is still
waiting for the permission from the Ministry of Posts and Telephones. Bangladesh
Railway Optical Fiber Networks is shown in Figure 2.1.
2.3 Fiber Optic Backbone Links in BTCL
The Telegraph branch of the Posts and Telegraph Department was created
in 1853 in the then British India, which was afterwards regulated under the
Telegraph Act of 1885. This was
Figure 2.1. Bangladesh
Railway Optical Fiber Networks.
reconstructed in 1962 as Pakistan Telegraph and Telephone Department.
After the independence of the People’s Republic of Bangladesh in 1971, Bangladesh
Telegraph and Telephone Department was set up under the Ministry of Posts and
Telecommunications to run the Telecommunication Services in Bangladesh. This
was converted into a corporate body named Bangladesh Telegraph and Telephone
Board after promulgation of Bangladesh Telegraph and Telephone Board Ordinance
No. XLVII of 1975. In pursuance of Ordinance No. XII of 1979 promulgated on 24th
February 1979, Bangladesh Telegraph & Telephone Board was again converted
into a Government Board. Then Bangladesh T & T Board (BTTB) was running as
a Government establishment under the Ministry of Posts and Telecommunications
(MOPT). From 1st July 2008 BTTB was converted into a limited company
which is 100% owned by the government.
BTCL’s Nationwide Optical Transmission Telecom Backbone
Infrastructure
Bangladesh is a revering country and the country’s long route
transmission systems were mainly composed of microwave, UHF and VHF radio
links. The use of optical fiber was first introduced by BTTB in some city areas
for interconnecting local exchange and Remote Switching Units (RSU) in Multi
Exchange Network. PDH technology was used in the early Optical links of BTTB. Optical Fiber Cable Network of BTCL is shown
in Figure 2.2.
However, BTTB planned to improve the quality and bandwidth of the long
distance transmission network by introducing optical fiber instead of
microwave. First they took a project to lay optic fiber cable in
Dhaka-Comilla-Feni-Chittagong route in 1997 with the help of French government.
Some spur links such as Comilla-B. Baria, Feni-Begumganj-Lakshmipur,
Begumganj-Maizdi and Dhaka-Gazipur was also included in the project. SDH
technology was first introduced in BTTB through this project. The project was
implemented with in 1999-2000. By the end of 1999 BTTB took a huge project
financed by China to digitalize its’ telephone exchanges as well as to improve
the transmission links of BTTB. Around 975 km optic fiber cable was laid at
different parts of Bangladesh under that project. Optic cable laid under the
project covered mostly the northern part of the country, from Bogra to
Panchagarh. Besides, some optic cable was laid in Mymensingh, Kushtia,
Rangamati, Pabna and Satkhira area under that project. Dhaka city was optically
connected to Bogra and Rajshahi (via Gazipur, Mymensingh, Tangail, Sirajganj
and Natore) under another project (266 KL project) with in 2006-2008. In July
2008 BTTB was transformed into a Public Limited Company and named as Bangladesh
Telecommunications Company Limited (BTCL). In the year 2008 a project named
“Info-bahan” was started in BTCL to improve the
Figure 2.2. Optical Fiber Cable Network of BTCL.
Internet
Network infrastructure of BTCL. A considerable amount of the project is
financed by Korean government loan. There is provision for laying of 1450 km
optic fiber cable under the project at different parts of the country of which
almost 360 km has been completed so far. Others are expected to the completed
by the end of June 2011. Three major cities (Khulna, Barisal & Sylhet) and
17 district cities will be connected through optic fiber under the project.
Besides, some important Upazilas will also be optically connected under the
project. Major optic fiber links of BTCL are shown in the following table 2.1.
Table 2.1 Long Distance Optical Fiber Networks of
BTCL
|
SL. |
Name |
Length |
No. |
Remarks |
||
|
1. |
Dhaka |
103 |
18 |
|
||
|
2. |
Comilla |
68 |
18 |
|
||
|
3. |
Feni |
102 |
18 |
|
||
|
4. |
Chittagong |
100 |
24 |
|
||
|
5. |
Chiringa |
68 |
24 |
|
||
|
6. |
Comilla |
81 |
12 |
|
||
|
7. |
Feni |
33 |
12 |
|
||
|
8. |
Begumganj |
30 |
12 |
|
||
|
9. |
Begumganj |
10 |
12 |
|
||
|
10. |
Dhaka |
38 |
12, |
Two |
||
|
11. |
Gazipur |
51 |
18/12 |
|
||
|
12. |
Bhaluka |
47 |
12/18 |
|
||
|
13. |
Mymensingh |
49 |
18/12 |
|
||
|
14. |
Modhupur |
50 |
12/18 |
|
||
|
15. |
Tangail |
47 |
18 |
|
||
|
16. |
Sirajgonj |
73 |
18 |
|
||
|
17. |
Mymensingh |
72 |
12 |
|
||
|
18. |
Mymensingh |
42 |
12 |
|
||
|
19. |
Sirajgonj |
50 |
12 |
|
||
|
20. |
Shahjadpur |
74 |
12 |
|
||
|
21. |
Pabna |
63 |
12 |
|
||
|
22. |
Kushtia |
59 |
12 |
|
||
|
23. |
Meherpur |
30 |
12 |
|
||
|
24. |
Bogra |
59 |
12 |
|
||
|
25. |
Palashbari |
58 |
12 |
|
||
|
26. |
Rangpur |
45 |
´ 2 |
|
||
|
27. |
Sayedpur |
19 |
12 |
|
||
|
28. |
Sayedpur |
46 |
12 |
|
||
|
29. |
Dinajpur |
60 |
12 |
|
||
|
30. |
Thakurgaon |
39 |
12 |
|
||
|
31. |
Rangpur |
43 |
12 |
|
||
|
32. |
Rangpur |
57 |
12 |
|
||
|
33. |
Khulna |
65 |
12 |
|
||
|
34. |
Chittagong |
16 |
24 |
|
||
|
35. |
Hathazari-Betbunia |
25 |
24/12 |
|
||
|
36. |
Betbunia |
34 |
12 |
|
||
|
37. |
Dhaka |
19 |
12 |
Part |
||
|
38. |
Bogra-Natore |
72 |
24 |
|
||
|
39. |
Natore-Rajshahi |
51 |
24 |
|
||
|
40. |
Dhaka-Sreenagar |
33 |
24 |
|
||
|
41. |
Sreenagar-Munshiganj |
35 |
24 |
Under |
||
|
42. |
Kushtia-Jhenaidaha-Magura-Jessore-Khulna |
185 |
24 |
|
||
|
43. |
Magura-Faridpur-Madaripur-Barisal |
190 |
24 |
|
||
|
44. |
B.Baria-Habiganj-Maulavibazar-Sylhet |
192 |
24 |
Under |
||
|
45. |
Khulna-Bagerhat-Gopalganj |
80 |
24 |
Under |
||
|
46. |
Tongi-Narshingdi |
40 |
24 |
Under |
||
|
47. |
Barisal-Jhalokathi |
22 |
24 |
Under |
||
|
48. |
Satkania-Bandarban |
22 |
24 |
|
||
|
49. |
Tangail-Mirzapur-Kaliakoir-Konabari-Gazipur |
75 |
24/12 |
Under |
||
|
50. |
Sherpur-Jamalpur |
21 |
24 |
Under |
||
|
51. |
Palashbari-Gaibandha |
22 |
24 |
Under |
||
|
52. |
Sylhet-Tamabil |
51 |
24 |
Under |
||
|
53. |
Kurigram-Ulipur |
19 |
24 |
Under |
||
|
54. |
Patia-Chandanaish |
8 |
24 |
Under |
||
|
55. |
Dinajpur-Fulbari |
45 |
24 |
Under |
||
|
56. |
Narshingdi-Bhoirab |
37 |
24 |
Under |
||
|
57. |
Chuadanga-Darshana |
26 |
24 |
|
||
|
58. |
Comilla-Chadpur-Kachua |
|
|
Under |
||
|
59. |
Mymensingh-Kishorganj |
75 |
24 |
Under |
||
|
60. |
Bagerhat-Pirojpur |
30 |
24 |
Under |
||
|
61. |
Jessore-Narail |
40 |
24 |
Under |
||
|
62. |
Bogra-Naogaon |
65 |
24 |
Under |
||
|
63. |
Bogra-Joypurhat |
60 |
24 |
Under |
||
|
64. |
Faridpur-Rajbari |
35 |
24 |
Under |
||
|
65. |
Gopalganj-Tungipara |
18 |
24 |
Under |
||
|
66. |
Gopalganj-Kotalipara |
16 |
24 |
Under |
||
|
67. |
Maulavibazar-Sreemongal |
20 |
24 |
Under |
||
|
68. |
Jessore-Jhikorgacha-Sharsha-Benapol |
50 |
24 |
Under |
||
|
69. |
Narail-Lohagarah |
20 |
24 |
Under |
||
|
70. |
Magura-Sreepur |
10 |
24 |
Under |
3693 km
|
2.4 Fiber Optic Backbone Link in Power Grid Company of Bangladesh
The Govt. of Bangladesh formed the Power Grid Company of Bangladesh
Ltd. (PGCB) in 1996 as a functional unit under the Power Sector Reform Program
and entrusted with the responsibility for operation, maintenance and expansion
of high voltage power transmission network of Bangladesh.
From the beginning of its inception, the company has undertaken
necessary progressive steps, so that the power generated from the power plants
could be evacuated and transmitted effectively and economically to the power
distribution sectors and ultimately to the doorsteps of the common people.
2.4.1 Present Communication System
For day to day routine works and system operation the company maintains
its own communication system called Power Line Carrier (PLC) communication
using its high voltage transmission lines connecting all the power plants and
high voltage sub-stations (230 KV& 132 KV) with the Load Dispatch Center,
denoted this:
- The functions include
Telecommunication, Tele-protection, Data-transmission, supervisory control
and data acquisition (SCADA), economic load dispatching etc. are being
done by the system.
- The frequency band of
the existing PLC communication system is very low (100 KHz to 500 KHz) and
its speech band is only 4 KHz.
- With this narrow
band-width, all the functions of system operation can not be coped-with
and its economic system operation is hampering badly.
2.4.2 Present Status of Optical Network of Power Grid Company of
Bangladesh
To overcome the low bandwidth problem, the company has decided to
expand its communication system and started installation of optical fiber over
its high voltage transmission lines throughout the whole country.
Until now 448 Km optical fiber (single mode) has already been installed
around Dhaka and Dhaka to Chittagong route as OPGW (Optical Ground Wire)
without having any terminal equipment. [9].
The route includes:
·
Ashuganj –
Comilla – Feni – Hathazari (with 8 fibers) as shown in annexure –1.
·
Hasnabad–Meghnaghat–Comilla–Haripur–Rampura-Ghorasal
(with 12 fibers).
·
Tongi-Aminbazar-Hasnabad
(with 48 fibers).
Existing Optical fiber network of PGCB is shown in Figure 2.3 [9].
Figure 2.3.
Existing Optical Fiber Network of PGCB.
2.4.3
Projects Under Taken of Power Grid Company of Bangladesh
With
a view to improve overall performance and efficiency of PGCB the Company has
undertaken different transmission line projects (230 KV & 132 KV).
All
the new lines will have provision of Optical Ground Wire (as OPGW) which will
facilitate spreading the back bone communication throughout the whole country.
Under
the project about 782 Km 230 KV and 527 km 132 KV transmission line will have
Optical Ground Wire, which includes:
- Ashuganj – Shahjibazar
– Srimongol – Sylhet 230 KV transmission line (170 Km). - Khulna – Bheramara –
Ishurdi –Baghabari – Sirajganj – Bogra –Bara pukuria 230 KV transmission
line (447 Km). - Ashuganj – Sirajganj
230 KV transmission line (165 Km).
- Rangpur – Bara pukuria
– Saidpur 132 KV transmission line (80Km).
- Mymensingh – Joydebpur
– Kabirpur – Tangail – Madhupur – Jamalpur 132 KV transmission line (447
Km), will have optical fiber as optical ground wire.
2.5
National
Load Dispatch Center Project
With
an objective to install a modern SCADA and Energy Management System (EMS) and
to develop reliable communication network to cover all the power stations and
grid sub-stations, the company has undertaken a National Load Dispatch Center
(NLDC) project to be located at Rampura (near Dhaka). In this project about
1200 Km optical fiber will be installed as ADSS (All-Dielectric
Self-supporting) cable and 151 km OPGW on existing transmission lines
throughout the whole country [11]. The
project includes:
- Comilla – Feni –
Hathazari – Madanhat – Shikalbaha – Dohazari – Cox’sBazar 132 KV
transmission line (288 Km) will be equipped with optical cable (ADSS).
- Bheramara – Faridpur –
Madaripur – Barisal– Patuakhali 132 KV transmission line (275Km) with
ADSS.
- Bogra – Natore –
Rajshahi – Chapai Nawabganj 132 KV transmission line (153 Km) will have
optical cable as ADSS and many other links required for connecting the
important locations.
- Bheramara-Botail-Jhenaidha-Jessore-Noapara-Goalpara-Bagerhat-Mongla
132 KV transmission line (180Km).
- Ashuganj-Ghorasal-Tongi-Kabirpur-Manikganj
230 KV transmission line (170Km).
- 71 Km OPGW on existing
230 KV transmission lines between Tongi – Ghorashal – Ashuganj by
replacing one of the earth wires.
- 80 Km OPGW on existing
132 KV transmission lines from Bheramara-Khulna s/s replacing one of the
wires.
·
45 Km Underground
optical cables in different location will also be installed.
Telecom
& RTU Divisions of PGCB is shown
in Figure 2.4.
It may be seen that after completion of the projects undertaken by PGCB
most of the district towns of Bangladesh could be connected at Cox’s Bazar with
the optical submarine cable is shown in Figure 2.5.
The Optical network of PGCB will be much reliable and safe in
comparison to other Optical Fiber Network presently working in the country
because PGCB‘s optical network has been installed over the high voltage
transmission line and so it is free from most of the unwanted interruptions
resulting from frequent cutting of soil due to construction works of different
departments.
PGCB will use some small portion (10%) of the optical fiber network
capacity and the remaining portion will remain spare after meeting different
needs of PGCB.
PGCB intends to lease out the rest (90%) through open tender to
interested party/parties engaged in Telecommunication, Internet service and
other ICT business .
The Optical network of PGCB would be very robust ‚ noise free and its
reliability will be over 99.98% and safe in comparison to other Optical Fiber
Network because this optical network has been installed over the high voltage
transmission line and so it is free from most of unwanted interruptions.
So the Optical network of PGCB can be comfortably used for
telecommunication, Internet service and other ICT business. PGCB will be highly
glad to invite national and international companies engaged in
telecommunication, Internet services and other ICT business to utilize the
latent capacity of the optical fiber installed by PGCB to make Bangladesh as a
part of Global Village.
 |
Figure 2.4.
Telecom & RTU Divisions of PGCB.
Figure 2.5. PGCB’s
future Optical network after completion of the projects under taken.
Optical Fiber Cable Network in Bangladesh Company
The government
will provide licenses for five more international connectivity cables-two
submarine and three terrestrial in the private sector. The cables will
facilitate the growth of information and communication technology (ICT) and
telecom sector through uninterrupted telecommunication services. Another
ministry official said the demand for bandwidth will rise in every sector in
Bangladesh, which now uses 17 gigabyte of its 44 gigabyte bandwidth capacity,
as the government aims to bring the whole country under fast internet connectivity.
The new five licences would be issued under guideline prepared by the
Bangladesh Telecommunication Regulatory Commission (BTRC).
3.1 Fiber Optic Backbone Link in Grameenphone
Before Grameenphone’s inception, the phone was for a
selected urbanized few. The cell phone was a luxury a flouting accessory for
the select elite. The mass could not contemplate mobile telephony as being part
of their lives.
Grameenphone started its journey with the
Village Phone program: a pioneering initiative to empower rural women of
Bangladesh. The name Grameenphone translates to “Rural phone”. Starting its
operations on March 26, 1997, the Independence Day of Bangladesh, Grameenphone
has come a long way. Grameenphone pioneered the then breakthrough initiative of
mobile to mobile telephony and became the first and only operator to cover 98%
of the country’s people with network.
Since its inception Grameenphone has built the
largest cellular network in the country with over 12,000 base stations in more
than 6000 locations. Presently, nearly 98 percent of the country’s population
is within the coverage area of the Grameenphone network. Grameenphone has
always been a pioneer in introducing new products and services in the local
market. GP was the first company to introduce GSM technology in Bangladesh when
it launched its services in March 1997.
Grameenphone was also the first operator to
introduce the pre-paid service in September 1999. It established the first
24-hour Call Center, introduced value-added services such as VMS, SMS, fax and
data transmission services, international roaming service, WAP, SMS-based
push-pull services, EDGE, personal ring back tone and many other products and
services. The entire Grameenphone network is also EDGE/GPRS enabled, allowing
access to high-speed Internet and data services from anywhere within the
coverage area. There are currently nearly 3 million EDGE/GPRS users in the
Grameenphone network.
Today, Grameenphone is the leading
telecommunications service provider in Bangladesh with more than 27 million
subscribers as of September 2010.
Grameenphone
- Grameenphone has so far invested more than
BDT 15,260 crore to build the network infrastructure - Grameenphone is one of
the largest taxpayers in the country, having contributed more than BDT 16,600
crore in direct and indirect taxes to the Government Exchequer over the
years. - There are now more than
600 GP Service Desks across the country covering nearly all upazilas of 61
districts and 78 Grameenphone Centers in all the divisional cities - Grameenphone has more
than 4500 full and temporary employees. - 150,000 people are
directly dependent on Grameenphone for their livelihood, working for the
Grameenphone dealers, retailers, scratch card outlets, suppliers, vendors,
contractors and others.
It is a joint venture enterprise between Telenor (55.8%), the largest
telecommunications service provider in Norway with mobile phone operations in
12 other countries, and Grameen Telecom Corporation (34.2% ), a non-profit
sister concern of the internationally acclaimed micro-credit pioneer Grameen
Bank. The other 10% shares belong to general retail and institutional
investors. The technological know-how and managerial expertise of
Telenor has been instrumental in setting up such an international standard
mobile phone operation in Bangladesh. Being one of the pioneers in developing
the GSM service in Europe, Telenor has also helped to transfer this knowledge
to the local employees over the years.
The international shareholder brings technological and business management
expertise while the local shareholder provides a presence throughout Bangladesh
and a deep understanding of its economy. Both are dedicated to Bangladesh and
its struggle for economic progress and have a deep commitment to Grameenphone
and its mission to provide affordable telephony to the entire population of
Bangladesh.
3.1.1 Fiber Optic Network (FON)
Grameenphone
(GP) acquired the Optical Fiber Network of Bangladesh Railway (BR), back in
1997 through a lease contract for 20 (twenty) years from the date of signing.
Grameenphone
Nation Wide Telecom Backbone Infrastructure has been built on SDH Fiber Optic
Network (FON) and Microwave Radio Links. SDH Fiber Optic Network refers to a
group of fiber optic transmission rates that can transport digital signals with
different capacities.
Total
length of Grameenphone Optical fiber is now about 2000km (2004) and the maximum
capacity is STM-4 (622Mbps, 252 E1 PCM), ready for upgrading to STM-16 (2488
Mbps, 1008 E1 PCM). Grameenphone Optical
fiber map 2010 is shown in Figure 3.1.
3.1.2 Grameenphone Nation Wide Telecom
Backbone Infrastructure
Grameenphone
Nation Wide Telecom Backbone Infrastructure carries the traffic (voice &
data) from point to point. It consists of three main parts as given below:
- Local Network: has been built on PDH MW Radio links in 15GHz
& 23GHz frequency band. - Regional Network: has been built on PDH MW Radio links in 7.5GHz
frequency band. - Backbone Network: has been built on SDH Fiber Optic Network (FON)
and SDH MW Radio links.
Some
details of GP’s available Telecom Backbone Infrastructure for Nation Wide
Connectivity:
- Space: Primary 6 divisional Head Quarter. For remaining
sites out door solution should be applied for Last mile equipment - Base Station/Access Node :
Grameenphone has built the largest cellular network in the country with
over 10,000 base stations in more than 5700 locations. - Dist. /Thana Coverage- covering nearly all upazilas of
61 districts.Existing Fiber Optic backbone links of Grameenphone is
shown in Figure 3.2.Reasons
behind Using of Optical Fiber of GP- Nationwide coverage
- Minimum downtime for
non-protective circuits - Redundant Network
option & Cost-effective - Right of way in the
Bangladesh Railway Network - Public but yet private
enough to give best negotiated prices - Best possible option
available
Figure 3.1. Grameenphone
Optical Fiber maps 2011.
Figure 3.2.
Existing Fiber Optic Backbone Links of Grameenphone.3.2 Fiber Optic Backbone Link in Robi
Axiata (Bangladesh) Limited is a dynamic and leading countrywide
GSM communication solution provider. It is a joint venture company between
Axiata Group Berhad, Malaysia (70%) and NTT DOCOMO INC, Japan (30%). Axiata (Bangladesh)
Limited, formerly known as Telekom Malaysia International (Bangladesh),
commenced its operation in 1997 under the brand name Robi among the pioneer GSM
mobile telecommunications service providers in Bangladesh. Later, on 28th
March, 2010 the company started its new journey with the brand name Robi.Robi is truly a people-oriented brand of Bangladesh. Having the local tradition
at its core Robi marches ahead with innovation and creativity. To ensure
leading-edge technology, Robi has the international expertise of Axiata and NTT
DOCOMO INC. It supports 2G voice, CAMEL phase 2 and GPRS/EDGE service with high
speed internet connectivity. Its GSM service is based on a robust network
architecture and cutting edge technology such as Intelligent Network (IN),
which provides peace-of-mind solutions in terms of voice clarity.3.2.1 Communication System of Robi
Presently they are connected
with Chittagong zone, Khulna zone, Sylhet zone etc. through PDH and SDH
microwave link. The PDH and SDH microwave link is not reliable for their telecommunication
network. So they are planning to complete the optical fiber network between
Dhaka to Chittagong within 2007. Robi Optical Fiber Network in Bangladesh is shown
in Figure 3.3. Long Distance Optical Fiber Networks of Robi
is shown in Table 2.2.Table
3.1 Long Distance (Dhaka to Chittagong) Optical Fiber Networks of RobiSL.
No.Name
of the Optic sectionLength
of the section (Km)No.
of fibers in the OF cableCapacity
of the terminal equipmentType
& Brand of terminal equipmentRemarks
1
Brac
– Kachpur26
2
STM-64/
STM-16Alcatel
2
Kachpur-Gouripur
45
2
STM-64/
STM-16Alcatel
Two
links3
Gouripur-Alckahar
46
2
STM-64/
STM-16Alcatel
Two
links4
Alckahar-Falgunkara
44
2
STM-64/
STM-16Alcatel
Two
links5
Falgunkara-Lemua
40
2
STM-64/
STM-16Alcatel
Two
links6
Lemua-Wahidpur
48
2
STM-64/
STM-16Alcatel
Two
links7
Wahidpur-Fauzdarhat
48
2
STM-64/
STM-16Alcatel
Two
links8
Fauzdarhat-Faruk
Chamber20
2
STM-64/
STM-16Alcatel
Figure 3.3.
Robi Optical Fiber network in Bangladesh.3.3 Fiber Optic Backbone Link in Banglalink
Orascom telecom Bangladesh limited (“Banglalink”) is a 100%
owned subsidiary of orascom telecom holding S.A.E., Egypt, (“OTH”) in
Bangladesh. It was acquired by OTH in 2004, and after a complete overhaul and
the deployment of a new GSM network, its telecommunication services were
re-launched under the brand name Banglalink. When Banglalink began operations
in Bangladesh in February 2005, its impact was felt immediately: overnight
mobile telephony became an affordable option for customers across a wide range
of market segments.Banglalink success was based on a simple mission, “bringing mobile
telephony to the masses” which was the cornerstone of its strategy.
Banglalink changed the mobile phone status from luxury to a necessity and
brought mobile telephone to the general people of Bangladesh and made a place
in their hearts. The mobile phone has become the symbol for the positive change
in Bangladesh.This positive change that is quite correctly attributed to Banglalink,
has become the corporate positioning of Banglalink and is translated in their
slogan “making a difference” or “din bodol”. “Making a
difference” not only in the telecom industry, but also through its
products and services, to the lives of its customers. This corporate stance of
“making a difference” has been reflected in everything Banglalink
does.Banglalink attained 1 million subscribers by December 2005 and 3
million subscribers in october 2006. In less than two years which is by December
2007, Banglalink overtook Robi to become the second largest operator in
Bangladesh with more than 7.1 million customers. Banglalink currently has 14.22
million subscribers as of march 2010, representing a market share of 26%.Growth over the last years have been fuelled with innovative products
and services targeting different market segments, aggressive improvement of
network quality and dedicated customer care, creating an extensive distribution
network across the country, and establishing a strong brand that emotionally
connected customers with Banglalink.3.3.1
Communication System of BanglalinkPresently they are connected with Chittagong,
Rajshahi, Sylhet etc. through PDH and SDH microwave link. They are planning to
setup the optical fiber long distance telecommunication network between Dhaka
to Chittagong and Sylhet within 2008, is shown in Figure 3.4. Banglalink
Optical Fiber Network Infrastructure Nation (2011) & shown in Figure 3.5. Banglalink Optical Fiber Network
Infrastructure Dhaka (2011).
Figure 3.4. Banglalink Optical
Fiber Network Infrastructure Nation 2011 [14].
Figure 3.5. Banglalink Optical
Fiber Network Infrastructure Dhaka 2011 [15].Slab charge of Banglalink is shown in Table2.3.
Selling unit:
Capacity from fiber optic/microwave backbone
A discount may be considered depending on bulk, tenure of utilization,
data/voice and type/category of client and the route of lease/sublease.EL rent shall be calculated by giving a discount range up to 30%
EL (30 channel)
Table 3.2 Maximum monthly charge per kilometer
Slab
Slab in Kilometers
Maximum monthly charge*/Km
Remarks
1
0-50
bdt 250
Note to table-a
2
51-100
bdt 200
In addition to (1)
3
101-200
bdt 160
In addition to (2)
4
201-300
bdt 130
In addition to (3)
5
301-and above
bdt 95
In addition to (4)
Note to table – a: for any distance below 20 km, the maximum monthly
charge will be bdt 5000/-.Selling unit: STM-1 (63 EL)
·
Capacity from
fiber optic/microwave backbone·
The rental shall
be determined by multiplying the unit cost of 1 (one) EL by 63STM-1 rent shall be calculated by giving a
discount range up to 30% may be considered depending on bulk, tenure of
utilization, data/voice and type/category of the client and the route of
lease/sub-lease. This discount shall only be applicable in the same
point-to-point link.Selling unit: STM-4/8/16 (4 STM -1/8 STM -1/16 STM -1)
·
Capacity from
fiber optic/microwave backbone·
The rental shall
be determined by multiplying the unit cost of 1 (one) STM-1 by 4/8/16STM-4/8/16 rent shall be calculated by giving a discount range up to
30% may be considered depending on bulk, tenure of utilization, data/voice and
type/category of the client and the route of lease/sub-lease. This discount shall
only be applicable in the same point-to-point link.Special tariff:
·
Corporate
Social Responsibility (CSR) cases: in
special cases like CSR, Banglalink may provide special discounted pricing which
are not to be treated as reference by any other organization or lease.·
Cases of
national interest: in cases of
national interest, Banglalink may provide special discounted pricing on case to
case basis which are not to be treated as reference by any other organization
or lease .3.4 Fiber Optic Backbone Link in
CitycellCitycell (Pacific Bangladesh Telecom Limited) is
Bangladesh’s and South Asia’s pioneering mobile communications company and the
only CDMA mobile operator in the country. Citycell is a customer-driven
organization whose mission is to deliver the latest in advanced
telecommunication services to Bangladesh. The company offers a full array
of mobile services for consumers and businesses that are focused on the unique
needs of the Bangladeshi community. Citycell’s growth strategy is to integrate
superior customer service, highest standards of technology and a choice of
packages at affordable rates. The company operates a 24-hour call centre
with well trained operators to respond to customer queries. Citycell’s customer
service is open 7 days a week to ensure customers can access Citycell at any
convenient time.3.4.1 Communication System of Citycell
Presently they are connected
with Chittagong, Rajshahi; Sylhet etc. through PDH and SDH microwave link. They
have plan to setup the optical fiber long distance telecommunication network in
whole Bangladesh. PBTL Nationwide OFC & Microwave Backbone for Transmission
and Bandwidth Lease. Citycell Optical
Fiber Network Infrastructure Nation (2010) is shown in Figure 3.6.3.5 Fiber Optic
Backbone Link in Fiber@HomeUnder National Telecommunication Transmission Network (NTTN) license,
issued by Bangladesh Telecommunication Regulatory Commission (BTRC), Fiber@Home
is developing a completely secured & robust optical fiber backbone network
nationwide international standard. This network is made up of HDPE (High
Density Poly Ethylene) duct placed underground in metro areas.Starting its official journey on 7th January, 2009 and being financed
by local investors, Fiber@Home is destined to build a network up to the Upazila
head quarters of the country. Working towards that goal Fiber@Home has built
Underground optical fiber Metro Ethernet Network in multiple metropolitan
cities and already reached the major districts and upazilas with its networks.
This fiber optical transmission network will eventually become the major
backbone for all kinds of telecommunications and electronic entertainment
services that are the prime elements of modern life.Total Network Cover Approx.
1200 km. Fiber@home Optical Fiber network in Bangladesh is shown in Figure 3.7 .
Figure 3.6. Citycell Optical Fiber
Network Infrastructure Nation 2011.
-
Figure
3.7 Fiber@home Optical Fiber network in Bangladesh.
The
government owned operator BTCL (at that time it was BTTB) started to deploy
their long distance optical cable network in the year 2000. Private mobile
phone operator Grameenphone made lease agreement with Bangladesh Railway to use
their long distance optical cable network laid mostly along the railway track.
In the second half of 2000 decade Grameenphone and several other mobile
operators invested a huge to develop their individual long distance optic fiber
cable network. All these operators tried to connect as many important
cities/towns as possible which resulted in a linear or star type network with
center at Dhaka. But for a reliable optic fiber communication network ring-type
protection is essential. Moreover, several operators laid optic fiber cable in
the same route, even along the same side of the highway with small separation
among the optic cable of different operators. There was no co-operation among
the operators regarding the resource sharing. Rather, there was a competition
among them regarding the optic fiber cable network. One operator tried to reach
an important place with optic fiber before any other operator reach their. In
doing this the very important requirement of network protection was ignored by
the operators most of the times. Some mobile phone operators engaged themselves
into the business of selling transmission bandwidth over their OFC network
though they did not have the license for this business.4.1
Long Distance Optical Fiber Network DevelopmentIn
the year 2008 BTRC tried to stop this trend and issued National Telecommunication
Transmission Network (NTTN) license to a company for installing, operating and
maintaining long distance optical fiber cable network. The government owned
operator BTCL got the NTTN license by default. In the year 2010, another NTTN
license was issued to another company. Now, both the private NTTN licensee
(Fiber@Home and Summit Group) are developing long distance optic fiber cable
network across the country. The government owned operator BTCL also is
expanding their optic fiber cable network by bringing more cities/towns under
the OFC network. But the very important requirement of network protection by
ring type configuration is yet to be fulfilled. After issuance of NTTN license
the mobile operators are not allowed to do transmission bandwidth business,
rather BTRC issued a guideline for resource sharing among the operators under
which different operators may share their extra resources.The
comparison between different optical fiber cables users in Bangladesh are shown
in the table 4.1.Table
4.1 Comparison between different optical fiber cables users in Bangladesh.Sl
BTCL
Grameenphone
Banglalink
Robi
PGCB
1.
Length
of Cable3693
Km1600
Km Gp Developed,2000
km (RW)1900
Km317
Km2571
Km2.
Capacity
STM-4,
16, STM-64STM-4
STM
-16STM-4/8/16
STM-16,
STM-64STM-4,16
3.
Bandwidth
usage622
Mb/s, 2.5 Gb/s, 10 Gb/s622Mbps,
2488
Mbps622
Mb/s,2.5
Gb/s ,10
Gb/s622Mbps,
2.5
Gb/s4.
Types
of usage (Voice, Data…etc)Voice,
Data, EDGE/GPRSVoice,
Data, EDGE/GPRSVoice,
Data, EDGE/GPRSVoice,
Data, EDGE/GPRSData
5.
No.
of Fibers used12,
18, 24,32,48,12024
pair / 482
2
8,12,24,
32,486.
Type
and brand of equipmentSiemens,
HuaweiEricson,
Siemens, HuaweiSiemens
Alcatel
Nortel
7.
Network
TypeGSM
GSM
GSM
GSM
Data
8.
Perfections
type/modeExcellent
Good
Good
Good
Good
Most of the long distance optical fiber cable network
characterized as following:- Large volume cable routing by all the Optical
Fiber users causes the high deployment cost. - Multi type and multi vendor environment bring
complexity in a high dense public at small area to cover with modern
Optical Fiber facility. - Unwanted risk feature by parallel Optical Fiber
user during maintenance. - Mixed type network used by multi user to sense in
a common area for common business case and goal.
4.2
Limitation of Fiber Optic Cable NetworkMost
of the problems in the long distance optical fiber cable network are caused by
cable breakdown. Cable breakdown events generally occur because of the
following causes:·
Underground
Optical Fiber cable may get damaged by Road repairing or expansion or
renovation.·
Many Companies
follows same route for their respective Optical Fiber layout. Hence, cable
breakdown may occur during maintenance of the cable network of other company.·
Sometimes cable
breakdown occurs due to theft or sabotage.·
Cable breakdown
may also occur due to accident in the road, specially on the bridges or
culverts.·
Cable breakdown
sometimes occurs during development or maintenance works done by other utility
service providers.·
The obvious
consequence of long distance Optical Fiber cable breakdown is interruption
Optical Fiber optical transmission link.·
Though the
interruption period may be minimized through efficient maintenance but it
cannot be eliminated if the Optical Fiber cable network is not protected by
ring.·
But with present
trend of Optical Fiber cable network development in linear or star topology
link interruption caused by cable breakdown cannot be totally eliminated.·
The most
efficient way to eliminate link interruption is to deploy the Optical Fiber
cable network in ring or mesh topology.- Common or single
Optical Fiber Network with highly protected for all the operator can be
used which will increase the efficiency of uses by sharing the resources
also decreased the deployment cost so that end user may get benefit in
terms of quality service in lower price. - Network redundancy will be better if the overhead
fiber network is used as the redundant of underground Fiber networks. At
present P.G.C.B. in installing overhead Optical Fiber network in some part
of Bangladesh. In future they will install over-head OF network all over
Bangladesh. Fibers of P.G.C.B can be used to complete the protection ring
or mesh for the National Optic Fiber Network where ever possible. - Route of Optical Fiber cable networks should be
marked in such a way that people can easily become aware of it and can
take necessary measure to avoid any damage during implementation of any
development work nearby the network.
Conclusion
In this project, the optical fiber cable networks of
different telecommunication company technical advantages and disadvantages of
the present cable network structures were analyzed. For optimum and efficient uses of the optical
fiber cable networks the following suggestions can be considered:- It is necessary to control all activities of
Optical Fiber Cable Networks centrally by BTRC to minimize the
installation and maintenance cost. - The most efficient way to eliminate link
interruption is to deploy the Optical Fiber cable network in ring or mesh
topology. - Telecommunication operator will get STM-64
(10GB/s) for transmitting high speed data to connect 3G network if ring
topology is used with high bandwidth 400 GHz.
References
[1] JOHN M. Senior, “Optical Fiber Communications,
Principles and Practice,” Publisher: Prentice Hall International,
Hertfordshire, UK, 1992; 2nd Edition[2]
http://www.olson-technology.com/mr_fiber/fiber-history.htm[3]
http://www.olson-technology.com/mr_fiber/fiber-history.htm[4] Fiber-Optic Communications Systems, Third Edition.
Govind P. Agrawal[5] http://www.ece.umd.edu/~davis/optfib.html
[6] http://www.betelco.com/bd/bdstel/icee.pdf
[7] http://www.railway.gov.bd
[8] http://www.btcl.gov.bd
[9]
http://www.ari.vt.edu/internet/presentations/fSession1.5-PGCB.pdf[10] www.pgcb.bd.com
[11]
http://www.ari.vt.edu/internet/presentations/fSession1.5-PGCB.pdf[12] http://www.grameenphone.com/about-us
[13]http://www.grameenphone.com/about-us/corporate-information/ownership
structure[14] http://www.grameenphone.com
[15] http://www.robi.com.bd/index.php/page/view/92
[17]
http://66.96.206.2/~banglali/docs.php?id=about%20banglalink&docs_id=45[18] http://banglalinkgsm.com
[19]
http://66.96.206.2/~banglali/docs.php?id=about%20banglalink&docs_id=88[20] http://www.citycell.com/index.php?pageid=17
[21] http://www.citycell.com
[22] http://www.fiberathome.net/?q=content/who-we-are
[23] http://www.fiber@home.com
[24] Data Communications and Networking, Forouzan,
TATA McGRAW-HILL EDITION.